The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced; such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
The most common type of lithographic printing plate to which the present invention is directed has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which is exposed becomes hardened the plate is referred to as negative-working. In both instances the image area remaining is ink-receptive or oleophilic and the non-image area or background is water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area of film is hardened by the action of light while the area not struck by light is removed. The light-hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
The present invention provides a novel aqueous developer for negative-working lithographic printing plates. Most developer compositions for negative-working printing plates disadvantageously consist primarily of organic solvents which are offensive through their volatility and high concentration, pose toxic exposure hazards to the end user and are pollutants to the environment when evaporated into the air and/or discharged into public waterways. Presently there is a trend toward the use of aqueous developers. Typically, these aqueous developers also contain organic solvents at concentrations which are still environmentally harmful and toxic to the end user. The prime advantage is that the solvents are not odor-wise offensive. They have the disadvantage of having a strongly alkaline pH, cause redeposit on the plate and machine parts, and solubilize the coating residue which creates disposal problems and reduces the effective life of the developer in a processing machine. The present invention provides a composition which substantially alleviates these problems.
Aqueous alkaline developing compositions, for use with negative-working lithographic printing plates, which contain an organic solvent and a surface active agent are well known. Such developing compositions are described for example in U.S. Pat. Nos. 3,701,657, 3,707,373, 4,186,006, 4,308,340, 4,350,756, 4,395,480, 4,716,098, 5,035,982, European Patent No. 0 080 042, British Patent No. 1,515,174 and British Patent No. 2,110,401. As disclosed in U.S. Pat. Nos. 3,701,657, 3,707,373, 4,350,756, 4,395,480 and 4,716,098 it is also known to incorporate in such developing compositions an aliphatic carboxylic acid or salt thereof. Such developers are generally quite effective but suffer from disadvantages which have hindered their commercial utilization, for example, they may not be able to adquately desensitize residual coating which remains in the background areas of the printing plate.
One of the more serious problems which can afflict negative-working lithographic printing plates is inability of the developer to remove all residual coating from the non-image areas of the plate. When sufficient residual coating remains, a condition exists for background sensitivity to occur during the printing process. Minimally, the effect would be to increase the amount of water required in the fountain solution. Under more severe conditions, ink may adhere to the background and ultimately to the printed sheet, thereby resulting in a condition known as "tinting" or "toning." Under extremely severe conditions, there is so much ink in the background that it is referred to as "scumming."
Among the requirements for an effective developer are (1) that it be resistant to oxidation, (2) that it be capable of dissolution-type processing, i.e., that the photosensitive composition dissolves fully in the developing solution rather than breaking up in the form of particles, and (3) that it be effective in desensitizing the non-image areas.
A particularly advantageous negative-working lithographic printing plate is that described and claimed in copending commonly assigned U.S. patent application Ser. No. 918,868, filed Jul. 23, 1992, "Photosensitive Compositions And Lithographic Printing Plates With Reduced Propensity To Blinding" by John E. Walls. This plate comprises an imaging layer containing a diazo resin, an acid-substituted ternary acetal polymer and an unsaturated polyester. One of its major advantages is its ability to resist blinding. As is well understood in the art, the term "blinding" refers to inability of the image areas to adequately take up the printing ink. In the printing plates of the aforesaid U.S. patent application Ser. No. 918,868, an important feature contributing to the ability to resist blinding is the presence of the unsaturated polyester. However, these plates sometimes exhibit a yellow stain. The use of the unsaturated polyester is, in part, connected with the stain. There appear to be other, as yet poorly understood, factors contributing to the stain. The yellow stain is known to be an extremely thin but strongly adhering layer of diazo resin and polymeric binder which is not removed in the developing process when conventional developers are used. The intensity of the stain is proportional to the amount of residual coating.
It is toward the objective of providing a new and improved aqueous developer, for use with negative-working lithographic printing plates, which has improved desensitizing capability that the present invention is directed.